1. Field of the Invention
This invention relates to an input circuit for use in transmitting a signal to a circuit device operating in a high speed such as an I/O interface used for inputting/outputting of a signal between semiconductor devices, or an I/O interface for inputting/outputting of a signal between a clock signal distribution circuit and each of the circuit components which are provided in an IC via a long wiring line.
2. Description of the Related Art
In an interface used in a circuit such as an ECL circuit for inputting/outputting of a signal, an open emitter circuit is generally used as the signal output section. Used as the signal input section, the input circuit having 50.OMEGA. terminal resistance and terminal voltage of -2 V. The specification of such an I/O circuit is determined by the voltage amplitude of the signal output section. Generally, the signal output section requires about 1 volt between the high and low levels of the signal. Accordingly, the interface between the signal input and output sections consumes a remarkably great amount of power.
In order to reduce the power consumption, the signal input section comprises an input circuit using a common-base circuit, i.e., a circuit in which the base is common to input and output circuits, in which a signal is input to an emitter. The input circuit can operate even if the voltage amplitude of the input signal is small. This is why it has been conventionally used to suppress the power consumption.
In a case of distributing a high-speed signal such as a clock signal to each circuit element in a high-speed large-scale integrated circuit, the conventional devices employ an interface system using a voltage amplitude. In this case, the scale of the IC and the wiring length in the IC increase, so that the resistance and parasitic capacitance of the signal line and the capacitance of the input circuit increases. Consequently, the voltage amplitude of the input signal decreases and the signal delay in the wiring increases.
In order to solve this problem, a high-speed transmission system is proposed, in which a signal is input in the emitter of the common-base circuit as a current-sense type circuit. Such a system has been conventionally suggested as a novel interface circuit. However, when the operation of the voltage generator connected to the base of the circuit is unstable and the voltage generator is apt to generate the noise, the input circuit may malfunction if the input signal has a small voltage amplitude. To prevent the malfunction of the input circuit, the voltage generator with high stability needs to be used as the voltage source in the circuit.